(1) Field of the Invention
The present invention relates to optical fixed attenuators that can be formed by axially joining the ends of two optical fibers by fusion splicing. The loss of the splice can be controlled which allows it to be used to attenuate the power of an optical signal in a transmission line. The present invention relates to the process and apparatus for producing this attenuator.
(2) Description of the Related Art
An optical communication network normally consists of multiple fiber lines in each of which one or more individual optical signals are transmitted. If more than one wavelength of light is used to transmit multiple signals through a single fiber, the technique is referred to as wavelength division multiplexing (WDM). Recently, WDM systems having in excess of 100 individual signals have been employed in order to increase the transmission capacity. Such systems require precise balance between the transmitted powers of the individual signals. Discrete optical fixed attenuators are the technology of choice for the purpose of balancing this transmitted power. Furthermore, the total optical signal power transmitted in one optical fiber has increased because the total optical power is the sum of the power contained in the various optical signals that are transmitted in that optical fiber. This can leave the system susceptible to signal degradations caused by nonlinear optical effects. Attenuators are installed in a transmission line to minimize or eliminate these effects.
Based on previous technologies, there are at least common five types of optical fixed attenuators as follows:
    1. An optical fixed attenuator in which two optical fibers are fusion spliced by a technique which controls the transverse core offset, as disclosed in Japanese Patent Application Laid-open No. H7-49422, H8-122531, H9-145928 and H10-123326;    2. An optical fixed attenuator representing an improvement on type (1) in which an additional electric discharge is applied to the fusion splice point, as disclosed in Japanese Patent Application Laid-open No. 2000-131625;    3. An optical fixed attenuator in which a different material having different light transmittances or a different fiber having different characteristics is inserted between two optical fibers, as disclosed in Japanese Patent Application Laid-open No. H7-104130, H10-133021 and H10-282341;    4. An optical fixed attenuator in which light absorbing transition metal ions (for example, Co2+) are doped in an optical fiber core; and    5. An optical fixed attenuator in which the forward distance during splicing of an optical fiber is controlled on a basis of a correlation that exists between the forward distance and splice loss, as disclosed in Japanese Patent Application Laid-open No. H11-305046.
However, each of the techniques for the producing the optical attenuator as described above has one or more of the following drawbacks:    Process 1: it is necessary to control the transverse optical fiber core offset between the optical fibers precisely in order to obtain the desired optical attenuation;    Process 2: the attenuator produced by this method has a high wavelength dependence.    Process 3: it is necessary to perform two splices in order to insert the different fiber type;    Process 4: it is necessary to dope the fiber with light absorbing transition metal ions in a high enough concentration in the optical fiber core in order to achieve the desired optical attenuation; and    Process 5: The attenuator produced by this method has a high wavelength dependence.